Stop-on carrier tuner



May 24, 1949. w. P. BOOTHROYD ETAL 2,470,843

STOP-ON CARRIER TUNER Filed Oct. 4, 1944 03 mww w m w r O W m f. A

n m 4 D w M Patented May 24, 1949 UNITED STATES PATENT OFFICE STOP-ON CARRIER TUNER Wilson P. Boothroyd and William H. Forster,

Philadelphia, Pa., assignors, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application October 4, 1944, Serial No. 557,201

The present invention relates to automatic frequency control circuits and particularly to means for causing an automatic frequency controlled local oscillator to sweep over a band of frequencies and to cease its sweeping action when a carrier signal within the desired band is received.

More particularly the invention relates to an similar to that of the copending application of Wilson P. Boothroyd, erial No. 557,200, and differs therefrom in that the present circuit provides a sine wave sweep circuit for controlling the local oscillator frequency rather than the sawtooth wave characteristic of the oscillator of the mentioned copending application.

It is an object of this invention to provide a sweep control for a local oscillator or converter of a superheterodyne receiver, which will cause the receiver to sweep over the band on which a number of transmitters may be operating.

It is another object of this invention to provide a sweep control for a local oscillator as described above in which the sweep generator is constant in operation over a range of applied grid voltages.

It is a further object of the invention to provide a sweep circuit generator which cuts off sharply at a specific bias voltage.

It is a still further object of this invention to provide a circuit in which the sweep generator will cut off in the presence of a received carrier within the band and the local oscillator will thereafter be automatic frequency controlled to pro,-

vide a continuous frequency to properly heterodyne with the particular receiver carrier wave.

Further objects and features of the invention will be apparent when the following description is considered in connection with the annexed drawing in which:

The single figure of the drawing is a schema-tic diagram of the sweep circuit of our invention. In this diagram the circuit is shown as applied to a superheterodyne receiver adapted for the reception of frequency modulated signals, particularly to a radar communications receiver. bviously. however, the circuit may be utilized in connection with any superheterodyne receiver.

Referring now to the drawing it will be seen that there is provided a normal form of automatic frequency control comprising a direct cur- 1 Claim. (01. 250-) rent amplifier tube ll controlled by the direct current from the discriminator 12. The output of the D. C. amplifier l I is applied to the repeller electrode of the local oscillator tube [0 which tube is in the present instance, a velocity modulated McN-ally tube, such for example, as the type 2K28. The circuit thus far described is identical with the automatic frequency control circuit of the copending application of Wilson P. Boothroyd above mentioned. The operation thereof is well known in the radio art.

In order to provide a sweep voltage our invention comprises a phase shift oscillator 13 having a phase shifting network generally designated M, the input grid of the oscillator being connected by means of the conductor l5 to the grid of the limiter in substantially the same manner as is shown in the above mentioned copending application.

In such a phase shift oscillator the amplitude of oscillation is a function of the grid bias as developed by the limiter grid. By proper circuit constants the oscillator may be adjusted so that, over a bias range, the amplitude is substantially constant, although the wave form becomes less distorted with higher bias. Furthermore, the tube l3 will cut off abruptly at a predetermined voltage. In addition, the cut-off transient has an approximately exponential decay wherefore there is no large transient to disturb the automatic frequency control lock-in. Thus the effect is one of adding to the cut-oil. sensitivity as compared for example with the relaxation sweep generator shown in the above mentioned copending application.

In the particular application which has been made of the circuit the phase shift oscillator l3 was designed to operate at about cycles per second. The value is of course arbitrary but the frequency must be, in any event, below the audio band and should be made as high as is consistent with this condition.

In the particular embodiment shown (in which the circuit, as has been mentioned, is applied to a radar communication system) the amplitude of oscillation of the generator l3 was approximately volts-peak-to-peak, this being determined by the B-plus voltage of 200 which was available. The amplitude of oscillation is a function of the grid bias developed by the limiter grid. It was found that with the particular oscillator in question the amplitude was substantially constant from a bias range of about 1.5 to about 2.7 volts, the wave form becoming less distorted with higher bias. It was likewise found that with 3 volts oscillation ceases. It is clear that the large range of bias over which there is substantially no change in operating voltage is extremely valuable and that the fact that the waveform of the sweep voltage may be somewhat distorted makes little, if any, difference in operation.

In the particular radar communications system for which the circuit was specifically designed, the noise in the high gain intermediate frequency system regularly produces between 1 and 2 volts of limiter grid bias. Also one volt of cathode bleeder bias is supplied by the resistor I 6 in the screen bleeder circuit. As a result the sweep wave oscillates at 150 volts of amplitude for any normal intermediate frequency noise level and, depending on the noise level, cuts off with from 2 to 1 volts of signal applied on the limiter grid.

Thus the receiver, when there is no carrier wave being transmitted within the band width, is caused to have its local oscillator or converter swept over the required band by virtue of the operation of the sweep oscillator or generator l3 as described. Upon the reception of a signal within the band, oscillator l3 cuts off and the automatic frequency control circuit takes control and locks the local oscillator i to the carrier frequency, to thereby produce a proper heterodyne frequency for the intermediate frequency amplification stages.

There is shown in the drawing at I! a neon tube which serves as an indicator of the operation of the oscillator l3. The tube i1 is coupled to the plate of the oscillator l3 by a condenser IB and is provided with bias from a bleeder comprising the resistors 20 and 2|. If the neon lampis one that flashes on a 60 volt potential, resistors 20 and 2! are adjusted to provide 55 volts of fixed bias. Thus the lamp will flash on a 5 volt peak A. C. signal and the visual indication will then serve to notify the operator that the sweep oscillator I3 is functioning.

file of this patent:

While we have described a. preferred embodiment of our invention, it is obvious that other embodiments and constructions may be adopted without departing from the spirit of our invention and, consequently, we desire to be limited not by the foregoing description but solely by the scope of the appended claim.

What is claimed is:

In a superheterodyne receiver adapted to receive signals within a band of frequencies and having a local oscillator for supplying heterodyne frequencies, means for automatically sweeping the tuning of the receiver over the frequency band comprising, in combination, an electron tube oscillator, means to apply a voltage to the control rid of said oscillator to cause said tube to oscillate, said voltage being derived from the receiving circuit and being proportional to the normal noise level therein, means to apply the output of said oscillator to the control circuit of the heterodyne oscillator to thereby cause the receiver tuning to sweep over the frequency band and means operable upon the reception of a signal within the band to increase the voltage applied to the control grid of said sweep oscillator and thereby cause said oscillator to cease to operate.

WILSON P. BOOTHROYD. WILLIAM H. FORSTER.

REFERENCES CITED The following references are of record in the UNITED STATES PATENTS Number Name Date 2,132,654 Smith Oct. 11, 1938 2,174,566 Case Oct. 3, 1939 2,231,806 Goldsborough Feb. 11, 1941 2,283,523 White May 19, 1942 2,287,925 White June 30, 1942 2,410,075 Hutchins Oct. 29, 1946 

